Landfill space is dwindling, industrial and municipal waste production is increasing, and the disposal of solids from waste water treatment is becoming more complex. Composting is a viable alternative for disposal of organic materials from nearly all waste streams. Composting can be used to process municipal waste water biosolids, to remediate industrial waste water biosolids, and to treat wastes from processing food and agricultural products. Composting can reduce the volume of organic waste materials by about 50% or more while producing a stable non-odorous material that can be used as fertilizer or an amendment for soil.
Before about 1970, composting was a simple process in which waste materials were piled and allowed to sit until they decomposed. It was typically done on a small scale and was not considered for industrial-scale problems. Initially, industrial-scale composting was viewed as a means to reduce the volume of a waste stream. Material was shredded to small bits and then composted to be reduced into even smaller particles. Grinding the material to be composted was considered advanced technology.
The next advance in composting technology came from the realization that the efficiency of composting could be increased by adding air to the composting mixture. The microbes that produce compost require air and will smother inside of a static unaerated pile. Hence, the initial methods of aeration involved moving the compost to allow air into the stack.
A typical example of this aeration is a windrow turner that picks up the compost and dumps it to one side. Process control was quite primitive. Approximately 95% of municipal composting sites are currently windrow turner operations. Piles were typically turned at the convenience of the operator, rather than to optimize the composting process. A typical pile of compost will use all of its oxygen within about 1/2 hour, so such windrow turning was seldom related to actual oxygen demand. Turning was done seldom enough that the microbes in the center of the pile were rapidly depleted and the center of the pile was no longer composting. Turning the pile merely reinoculated the material from the center with fresh microbes and composting continued in the center of the pile for another 1/2 hour when the oxygen supply was depleted.
In addition, composting with windrow turners was typically done in an open unsheltered area. The vagaries of the weather and rainfall most often determined the water content of the composting mass. When there was too little rain, the pile was often too dry. When there was too much rain, the pile was wet and required frequent turning. Too much rain could also lead to problems with runoff of leachate.
One method used to overcome some of the disadvantages of pile composting is to enclose the piles in a building. A roof can keep rain off of the compost and allow better regulation of water content. However, such a facility is very expensive. Furthermore, it involves managing the quality of large volumes of air within the building. Operators must enter the structure where the atmosphere can be irritating, if not toxic, to maintain the composting process.
Some of the disadvantages of pile composting are overcome by in-vessel processes. Such processes enclose the compost in a reactor vessel. They can also mechanize materials handling and mixing of the organic materials to be composted. Vessel systems to date have typically been complicated systems which require precision construction techniques and permanent, stable foundations. Organic waste is typically fed into an opening at one end of the reactor and compost is removed from the other end. The material is moved through the reactor either by a complex moving floor apparatus or by an hydraulic ram. Aeration can be provided by pressurized air forced through the organic waste by air vents located throughout the moving apparatus. In-vessel reactors at least provide potential for collection of odorous emissions. The compost process can be enclosed and the air routed through a filtration system.
Some in-vessel systems also include mixing systems, typically rotating paddles or prongs, within the compost mass. Other in-vessel systems are static. The agitation systems used in in-vessel systems are expensive, prone to wear and failure, and provide agitation only at fixed intervals in the composting process.
U.S. Pat. No. 5,407,809 discloses an in-vessel system including a rotatable digester drum. The system includes an apparatus to rotate the drum. The drum has ribs and baffles that slow the flow of the organic material through the inclined drum and mix it as it progresses through the drum. All of the waste material progresses through the drum at the same rate. Air can be circulated through the drum to control the temperature of the composting mixture. Process control in this method includes the controlled rate of progression of the organic material through the drum and air circulation.
U.S. Pat. No. 5,312,754 discloses a container for composting that contains an inflatable bladder at the bottom of the container. At the start of the composting process, material is added to the container leaving enough empty space to allow inflation of the bladder. Inflation of the bladder lifts and then deflation lowers the compost to provide some mixing of the composting material. The system is portable and will work in commercial containers. Air circulation controls the temperature in the reactor and exhaust air is filtered to diminish odorous emissions. Process control includes mixing the original organic material by inflating the bladder and air circulation through the material.
U.S. Pat. No. 5,409,831 discloses a tunnel composting apparatus with a moving floor and mixing. Each of 14 trays are loaded at one end of the composter with up to 200 pounds of compost. A ram pushes the row of 14 trays along the floor of the composter. Aeration occurs in the tunnel middle, and there are two mixing stations in the center of the composter as well. Typically each tray takes 14 days to progress through the composter. This apparatus controls the composting process by mixing and aerating each tray of compost.
U.S. Pat. No. 4,798,802 discloses another moving floor in-vessel composting apparatus. This apparatus is a large, permanent installation in which compost is fed in one end of and moved through a two-level container. After the moving through the top level of the container, the compost is mixed and dropped into the lower level of the container, which is roughly a mirror image of the top level. The compost progresses through the second half of the container to the exit. Air is added to the compost through vents in the floor of the apparatus to allow temperature control. Process control includes possible feedback control of temperature and water levels, mixing half way through the process, and aeration.
U.S. Pat. No. 5,417,736 discloses an inexpensive, static, in-vessel composting system and a method for its use. A large plastic bag with a plurality of small vents is filled with compost and the bag is sealed. The organic material is pressed into the bag under pressure to eliminate shifting and to retain heat. The bag is penetrated by several ducts connected to an air supply. Air is forced through the ducts, into the compost, and out through the vents in the bag. As the air reaches the bag, moisture can condense on the bag resulting in retention of the condensate in the bag. The air flow can be used to control temperature.
U.S. Pat. No. 4,483,704 to Easter, II discloses a method for composting sludge by mixing the sludge with carbon material, layering the mixture in a silo and moving it through the silo on a timed interval basis. Aeration is possible in the silos. As additional composting sludge is acquired, it is further layered into the silo. The composting sludge is periodically withdrawn according to the timed interval basis and further layered in a second silo.
U.S. Pat. No. 5,190,572 to Long discloses a composting bin for residential use, the bin comprising stackable sub-units which can be assembled and disassembled. To use the bin, the sub-units are stacked to provide a first stacked bin, and biodegradable matter is placed into the bin to start the composting process. After substantial decomposition has occurred, the original top sub-unit is removed and placed on the ground to be the bottom sub-unit for a second stacked bin. The upper composting material is removed from the first bin and placed in the second bin. The next sub-unit is removed from the first bin and placed on the second bin, and more composting material from the first bin is placed in the second bin. This stacking procedure is continued until all the sub-units and composting material have been moved to the second bin. In this process of reversing the sub-unit stacking order, the compost is substantially mixed and aerated.
U.S. Pat. No. 5,354,349 to Inoue discloses a continuous method of producing compost by pulverizing the material and then carrying the material via conveyor belt through a treatment tank where it decomposes. The composted material can be returned to the process to be reprocessed, or can be mixed with unprocessed material and then processed. Inoue discloses an optimal moisture content in the mixture of 45-65 weight percent.
U.S. Pat. No. 4,339,265 to Engelmann discloses a method and apparatus for treating sewage including converting the solid sewage material into flat patties. The patties are stacked with wood chips as a bulking agent to serve as spacers and filler between the patties and the patties are allowed to decompose. The wood chips are separated from the decomposed material and can be reused as filler for the next patties.
U.S. Pat. No. 4,662,900 to Ottengraf discloses a method for removing malodorous and/or toxic components from gases by passing the gas stream through a filter bed. The filter bed consists of a mixture of an organic material, an inert fraction, and additives for neutralization and organic buffering. The entire bed, including the container, can be made of biodegradable materials.
There remains a need for an improved and economical in-vessel composting process in an industrial scale which is convenient, odorless, and not labor intensive.